The miniaturization of semiconductor devices such as DRAM (Dynamic Random Access Memory) has been accompanied by the creation of a need for capacitors that occupy a smaller area, these capacitors being the capacitive elements that make up a semiconductor device. A capacitor that makes up a semiconductor device is typically formed by providing a cylindrical hole in a silicon oxide film and then providing capacitor electrodes on the inner surfaces of the cylindrical hole. The capacitance of the capacitor that is formed is a value that depends on the surface area of the capacitor electrodes, i.e., the area of the inner surfaces of the cylindrical hole.
Reducing the size of the footprint of a capacitor entails decreasing the diameter of the cylindrical hole. If the diameter of the cylindrical hole is decreased without changing its depth, the area of the inner surfaces of the cylindrical hole decrease and the capacitance of the capacitor decreases. However, in order to allow the semiconductor device to operate correctly, a capacitor capacitance of at least a fixed value must be maintained.
In order to compensate for the amount of decrease in the capacitance of the capacitor that results from reducing the area occupied by the capacitor, a technique is typically employed in which the depth of the cylindrical hole that is used to form the capacitor is increased. According to this technique, a decrease in the capacitance of the capacitor that results from a decrease of the diameter of the cylindrical hole can be compensated for by increasing the depth of the cylindrical hole and thus increasing the area of the inner surfaces of the cylindrical hole.
In the technique described above, the smaller the area that can be occupied by the capacitor, the more the diameter must be decreased and the greater the depth must be increased in the cylindrical hole that is formed. The cylindrical hole is normally formed by erosion brought about by etching a silicon oxide film. When etching a small diameter, the diameter tends to decrease in the direction of the advance of etching. Accordingly, the depth that can be etched is determined by the size of the diameter, and forming a cylindrical hole having a smaller diameter and greater depth therefore becomes problematic, and forming a cylindrical hole of a desired depth becomes impossible.
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2006-245364) and Patent Document 2 (US 20030008469 A1) disclose a technique of forming a cylindrical hole in a silicon film in which the rate of advance of etching is faster than for a silicon oxide film. According to the technique disclosed in Patent Document 1, the rate of advance of dry etching of the silicon film is faster and the hole-opening process is easier than for a silicon oxide film, whereby, despite the smaller diameter, a cylindrical hole of greater depth can be formed.